The NHS in England could be in a funding crisis before the next general election, an influential think tank has said.

Accounts from 2013/14 show that the system is under "severe financial pressure", t he Nuffield Trust has said.

The independent health charity's latest report Into the Red states the financial strength of NHS trusts is "weak and declining".

There are an increasing number of NHS trusts in deficit even with hundreds of millions of pounds of additional funding, the authors said.

Provisional data for 2013/14 suggested that trusts were in a net overall deficit of more than £100 million compared with an overall surplus of £383 million the previous year, they said.

And 66 trusts across the country were now in deficit compared to 45 in 2012/13.

The authors said that the NHS had been through a period of "unprecedented" financial challenge.

The NHS had risen to the challenge of living within its means since 2010 but was now " increasingly poorly placed" to manage the impact of austerity, they said.

They concluded that NHS finances would continue to deteriorate further this year and next.

Meanwhile, more than 100 health and social care leaders raised concerns about the future sustainability of the health and social care systems.

Two-thirds of those polled by the Nuffield Trust said they felt that NHS providers would have to go into deficit in order to provide a high-quality service.

And half said they believed the health service would no longer be free at the point of use in a decade.

Andy McKeon, senior policy fellow at the Nuffield Trust and report co-author, said: "The NHS has risen to the challenge of living within its means over the past three years.

"But it has now reached a tipping point.

"Our analysis shows just how poorly placed it is to cope with the squeeze still to come.

"Demand for NHS services shows no signs of abating.

"With hospital finances increasingly weak, growing pressures on staffing, and the goal of moving care out of hospitals and into the community proving elusive, the NHS is heading for a funding crisis this year or next.

"As our panel of health and social care leaders suggests, the immediate choice is rapidly becoming one of financial deficits or scrimping on the quality of care.

"Too many hopes have been pinned on achieving radical system change quickly.

"Such changes take time and their impact is uncertain."

Health Minister Lord Howe said: " These predictions are pessimistic and paint an unrealistic picture of how our NHS is working. We know some parts of the NHS are under pressure due to an unprecedented rise in demand - which is why in very tight economic circumstances, we have taken tough decisions to increase the NHS budget by £12.7 billion over this Parliament."

Shadow health secretary Andy Burnham said: " Everywhere you look, there are signs of an NHS now heading rapidly in the wrong direction. It is not just standards of patient care that are getting worse but NHS finances are in a dire state.

"David Cameron needs to take urgent action to stabilise the deteriorating situation in the NHS. Patients seeing waiting times getting longer will fear much worse will be in store next year."

An NHS England spokeswoman said: "The NHS is under pressure, but patients are continuing to get high quality care, and satisfaction remains extremely high. Across the NHS there is now a great willingness to think creatively and radically about how to reshape care in new ways so as to future proof the health service for generations to come."